1. Field of the Invention
Embodiments of the specification generally relate to wireless communications, and more specifically to tracking timing changes with a differential phase shift keying (DPSK) wireless receiver.
2. Description of the Related Art
Differential phase shift keying (DPSK) modulation is often used for wireless applications where a simple modulation scheme is sufficient to support the amount of data to be carried. For example, the extended data rates (EDR) of Bluetooth version 2.0 and 2.1 as specified by the Bluetooth Special Interest Group (SIG) use 4 DPSK and 8 DPSK modulation schemes to support 2 Mbs and 3 Mbs of data throughput respectively. DPSK modulation is relatively easy to implement in both wireless transmitters and receivers.
As is well-known, with DPSK modulation, data is encoded with phase shifts between DPSK constellation points. The wireless transmitter transmits constellation points to the wireless receiver. The constellation points are typically sent periodically. For example, a constellation point transmission rate that supports the 3 Mbs Bluetooth EDR data rate may be 1 MHz (i.e., 1 us period). The wireless receiver determines phase differences between transmitted constellation points, and thereby determines the related transmitted data.
FIG. 1 is a diagram of an 8 DPSK system including eight constellation points 101, 103, 105, 107, 109, 111, 113, and 115. Data is encoded by phase changes between constellation points. For example, the phase change associated with going from point 101 to point 111 may be associated with a particular symbol. The wireless transmitter may send a series of symbols represented by phase transitions between multiple points on the constellation of FIG. 1 to the wireless receiver.
The wireless receiver processes a received signal to determine the transmitted constellation points. By determining the constellation point, the symbol, and thus the transmitted data, may be determined. Oftentimes, the received signal may not perfectly align with a constellation point; however, the symbol may still be determined by associating the received signal with the closest constellation point. Since the symbols are transmitted periodically, the wireless receiver may determine subsequent symbols by periodically examining the received signal to determine subsequent constellation points. Continuing the example of a 3 Mbs Bluetooth EDR signal, a symbol is transmitted approximately every 1 uS. Thus, the wireless receiver may determine the received constellation point approximately every 1 uS.
However, the timing relationship between the receiver and the received signal may change for a variety of reasons, such as temperature drift or oscillator inaccuracies at either the transmitter or receiver. A change in timing may result in the wireless receiver incorrectly determining the transmitted constellation point.
Therefore, what is needed in the art is a method to determine changes in timing of a received DPSK signal and compensate for the changes within the processing of the DPSK signal.